My SL brothers appear to have developed lock jaw regarding this question. I am throwing it out to the Tech Help group in hopes of some comments/suggestions

What would cause A) a Check Engine Light, and B) the engine to run lean to the point of a big jump in MPG?

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'92 MB 500 SL
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'98 Harley Davidson RoadKing
'74 BMW 3.0 CSi - Gone to a good home